There are more than 7 billion people in the world. As you read this sentence, at least three more people will be added. Think about that for second or so, and there’s another three. Why is the human population growing so fast? Has it always grown this fast? What causes populations to grow?

Human Population Numbers

Bacteria in a dish with ample nutrients double in population until there is no more space. Every population eventually reaches its maximum size. The population of bacteria in the dish reaches its carrying capacity. The
carrying capacity
is the number of an organisms that can live with the resources that are available. The
limiting factor
for the bacteria in the dish is space. That's the factor that limits the population of the bacteria in the dish. If there were unlimited space then maybe nutrients would be the limiting factor. Some resource will always run out to limit a population. Let's now apply these concepts to the human population.

Human population growth over the past 10,000 years has been tremendous (
Figure
below
). The entire human population was estimated to be:

5 million in 8000 BC

300 million in 1 AD

1 billion in 1802

3 billion in 1961

7 billion in 2011

Human population from 10,000 BC through 2000 AD. This shows the exponential increase in human population that has occurred in the last few centuries.

The Rate of Growth

Not only has the population increased, but the rate of population growth has increased (
Figure
below
). The population was estimated to reach 7 billion in 2012, but it did so in 2011. This was just 12 years after reaching 6 billion.

The amount of time between the addition of each one billion people to the planet’s population, including speculation about the future. Already, the population reached 7 billion one year earlier than projected.

As the human population continues to grow, different factors limit population in different parts of the world. Space, clean air, clean water, and food to feed everyone are limiting in some locations. Worldwide though, human ingenuity has not placed a limit on the population. Not only does the population increase, the rate of population growth increases.

There is a sign that population growth is slowing. Although population continues to grow rapidly, the rate that the growth rate is increasing has declined. Still, a recent estimate by the United Nations claims that 10.1 billion people will be sharing this planet by the end of the century. The total added will be about 3 billion people. That number is more than were even in existence as recently as 1960. Yet that is much less than the number we would expect if 1 billion people were being added every 12 years.

Summary

The human population is growing more than exponentially.

The rate of human population growth is still increasing. However, the rate at which the rate of growth is increasing has declined.

The United Nations estimates a population of 10.1 billion by the end of the century.